The process of crimping is widely used in the manufacture of, for example, electrical insulators and surge arresters.
In such technologies it is known to use a process known as “centered crimping” to manufacture electrical insulators.
In one form of this prior art method, an electrically insulating glass fiber rod is pushed into a center of a hollow, cylindrical, metal end-fitting having an open aperture. The aperture defines a clearance that is only slightly larger than a diameter of the glass fiber rod.
A metal wall of the end fitting is then crimped, or pressed under hydraulic pressure, onto the rod using hardened metal dies. The end fitting is as a result strongly bonded to the insulator rod. The bond between the components can withstand high forces, such as the tension and weight of overhead power lines in the span between adjacent structures in an electrical distribution network.
FIG. 1 shows a glass fiber rod 10, a cylindrical metal end fitting 11, and crimping dies 12 of the above-described centered crimping method, in which the dies 12 are moveable radially in directions of the arrows in order to effect deformation of the end fitting 11.
There are however requirements to crimp end fittings onto, for example, insulating rods, in an off-center manner.
Such requirements commonly arise in the manufacture of surge arresters. Surge arresters are used to protect equipment connected to power distribution networks from damage by excessive voltage situations caused by lightning strikes, switching surges, incorrect connections, and other abnormal conditions or malfunctions.
The active element in a surge arrester is a varistor, also referred to as a non-linear resistor because it exhibits a non-linear current-voltage relationship. If the applied voltage is less than a certain voltage (the switching or clamping voltage), the varistor is essentially an insulator and only a small leakage current flows through it. If the applied voltage is greater than the switching voltage, the varistor's resistance drops, allowing an increased current to flow through it. That is, a varistor is highly resistive below its switching voltage and substantially conductive above it. The voltage-current relationship of a varistor is described by the equation:
  I  =            (              V        C            )        α  
In the equation, I is the current flowing the varistor; V is the voltage across the varistor; C is a constant which is a function of the dimensions, composition, and method of fabrication of the varistor; and α (alpha) is a constant which is a measure of the non-linearity of the varistor. A large α, signifying a large degree of non-linearity, is desirable.
The surge arrester is commonly attached to an electrical power system in a parallel configuration, with one terminal of the device connected to a phase conductor of the electrical power system and the other terminal to ground or neutral. At normal system voltages, the surge arrester is resistant to current flow (except for the leakage current). If an over-voltage condition exceeding the switching voltage develops, the surge arrester becomes conductive and shunts the surge energy to a value while “clamping” or limiting the system voltage to a value which can be tolerated, without damage, by the equipment being protected.
The mechanical strength and integrity of the surge arrester can be achieved by assembling the core of the arrester from a single varistor element or a stack of varistor elements held between two end terminals by a plurality of elongate strength members disposed therearound. The ends of the strength members are inserted into recesses in the end terminals. Crimping of the end terminals distorts the recesses sufficiently to hold the strength members firmly therewithin (as disclosed in U.S. Pat. No. 5,680,289).
FIG. 2 is an exploded view showing the components of one type of surge arrester S.
In FIG. 2, the components of the surge arrester S when assembled together comprise four elongate glass reinforced polymer rods R that are at each end received in respective apertures located adjacent the corners of respective, essentially square end fittings F.
The end fittings F are crimped onto the rods R.
In the space between the end fittings F lies a series of cylindrical elements defining a varistor V of the aforementioned type. The assembly process for the surge arrester S is such that the rods R are under tension after crimping, which occurs while the end fittings F are compressed to press the components of the varistor V together.
This is achieved by way of the arrester S including in its structure one or more springs acting between the fittings F. The springs (which typically are disc springs) tend to lengthen the overall assembly. The rods R resist such lengthening.
Since the elements of the varistor V are contained within a cage defined by the rods R, the surface arrester S as a whole possesses good structural integrity.
However, as described below, excessive crimping during the manufacturing process crushes the glass fiber/matrix of the load-bearing members and greatly diminishes the mechanical performance of the product.
FIG. 3 shows the result of practicing the method of U.S. Pat. No. 5,680,289 on a cylindrical end fitting 16 during manufacture of a surge arrester of similar design to that shown in FIG. 2. In FIG. 3, a circular array of glass fiber rods 10 is inserted into a series of apertures 13 formed in an end face 14 of a cylindrical end fitting 16 that supports a stack of varistor elements 17.
In accordance with the method of U.S. Pat. No. 5,680,289, regions 18 of the exterior of the end fitting 16 are deformed by dies that are similar to the dies 12 of FIG. 1, in order to crimp the end fitting 16 onto the inserted ends of the other protruding rods 10 at each of the apertures 13.
The extent of the deformation in each of the regions 18 is essentially uniform along its length.
The gaps between adjacent dies used for forming the deformed regions 18 result in ridges 19 spacing the regions 18 from one another.
The example of FIG. 3 therefore relates to off-centered crimping, as compared with the centered crimping of FIG. 1. When attempting to use the known crimping apparatuses for off-center crimping, it becomes considerably more difficult, than when using the apparatus in the “centered” configuration of FIG. 1, to achieve a uniform gripping or crimping pressure acting around the circumference of the insulating rods 10 inserted into the apertures 13.
This problem manifests itself as undesirable peaks in the contact pressure acting on the load-bearing member. These can cause the aforesaid crushing of the glass fiber/matrix material of the load-bearing members.
The problem is illustrated schematically in FIG. 4, which shows in enlarged view two conventional dies 12 acting to effect off-center crimping of a rod 10 received in an aperture 13 of an end fitting 11, during manufacture of the FIG. 3 sub-assembly.
As is visible in FIG. 4, each of the dies 12 has a contact face 28. This causes deformation of the metal of the end fitting on advancing of the dies 12 into the end fitting 11.
Since each of the contact faces 28 is of essentially the same shape as the part of the periphery of the end fitting 11 that it engages, the deformation of the end fitting 11 is essentially invariant over the lengths of the periphery contacted by the contact faces 28. This in turn leads to unbalanced contact pressure acting on the rod 10 (as signified by the arrows in FIG. 4), thereby causing the aforesaid problems.
WO-A-01/15292 attempts to solve this problem in the manufacture of a surge arrester, by crimping onto the ends of the load-bearing members 10 respective, frusto-conical bracing cylinders. These may be applied using the center crimping method exemplified by FIG. 1. On assembly of the surge arrester, the bracing cylinders are received in tapered apertures in the end fittings, such that tension in the load-bearing members drives the bracing cylinders into the apertures.
This method of arranging the components of a surge arrester, however, only enjoys mechanical integrity while under tension. The surge arrester could be disassembled when compressed.
Furthermore, the need separately to crimp, at each end of each load-bearing member, a bracing cylinder adds to the complexity and cost of the manufacturing operation.
Other prior art methods of making surge arresters, involving the use of pressure screws to tension the load-bearing members and the forming of loops in the ends of the load-bearing members, are unacceptably complicated. Thus, there is a need for methods and apparatuses that improve the mechanical performance of products such as surge arresters, without compromising in terms of cost or complexity.